1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a trench-type power MOSFET (metal-oxide-semiconductor field-effect transistor).
2. Description of the Prior Art
One conventional semiconductor device will be described below with reference to FIG. 1 of the accompanying drawings. FIG. 1 shows a power MOSFET having a trench structure.
As shown in FIG. 1, the power MOSFET has a common N.sup.- -type drain layer 1 deposited on the surface of an N.sup.+ -type semiconductor substrate 8 by epitaxial growth, and a channel layer 2 that is formed in a surface layer of the common N.sup.- -type drain layer 1 by diffusing a P.sup.+ -type impurity therein. A source region 5 is formed in a surface layer of the channel layer 2 by diffusing an N.sup.+ -type impurity therein. The power MOSFET includes a trench defined vertically in the common N.sup.- -type drain layer 1, the channel layer 2, and the source region 5. The surface of the trench is covered with a gate insulating film 3, on which there is deposited a polysilicon gate 4 that fills the trench.
The polysilicon gate 4, the channel layer 2, and the source region 5 are covered with an interlayer insulating film 6 which has contact holes defined therein over the source region 5. An interconnection layer 7 is deposited on the interlayer insulating film 6 and in the contact holes in contact with the source region 5.
Heretofore, the conventional power MOSFET has been manufactured by a fabrication process that comprises the steps of (1) forming a guard ring, (2) separating a device region, (3) forming a channel layer in the device region, (4) forming a body region, (5) diffusing an impurity to form a source region, (6) forming a trench, (7) forming a gate electrode, (8) forming contact holes in an interlayer insulating film for connection to the source region, and (9) patterning an interconnection layer. In each of these steps, there has been required a photomask that is indispensable for a photolithographic process for patterning a deposited layer or layers. Therefore, the fabrication process has necessitated a total of nine photomasks.
These required photomasks and associated procedures for handling the photomasks have made the fabrication process highly complex, with the result that it is relatively expensive to manufacture the conventional power MOSFET.